Natural History magazine used the mural on its cover in September of 2001. It was my very first national magazine cover, but as fate would have it the tragedy of 9/11 happened and it got lost in the chaos of those times.
Dr. John Lundberg from the Academy of Natural Sciences wrote a wonderfully informative article about the reasons for the incredible fish diversity that accompanied the mural layout in the magazine. Here is the article:
Freshwater Riches of the Amazon
Natural History, Sept, 2001 by John Lundberg
To find the reasons for a river's abundance, a scientist goes fishing in deep time.
Home to more than two thousand freshwater fish species, the Amazon, ichthyologically speaking, is the hottest big river on the planet. Some of its inhabitants are familiar: cardinal tetras, discus, angel cichlids, and armored "pleco" catfish inhabit home aquariums; trophy-sized peacock bass and goliath catfishes beckon sporting anglers; giant-sized tambaqui and pirarucu are prime food fishes for residents of the region; and piranhas, electric eels, and river stingrays contribute to tales of tropical danger. But any notion that science has achieved a complete inventory of the Amazon's, or the world's, fishes is utterly dispelled by ongoing discoveries. Each year since 1960, more than 35 tropical American species of fishes, including catfishes, characins, electric fishes, killifishes, and cichlids, have been newly described and named. During the past decade the pace has quickened, with more than 50 fishes coming to scientific light annually. By contrast, the Congo River of Africa has about 700 fish species in total, and the well-studied Mississippi-Missouri of North America, a relatively scant 375.
I had surveyed the fishes of the Orinoco River system of Venezuela and Colombia for many years before my first visit to the Amazon in 1990. For an ichthyologist, the Amazon represents a pinnacle of diversity, but I also wanted to investigate the history and origins of the river's many fish species. Biogeography is the science that seeks to document and explain patterns of diversity and regional differences in species abundance, so I delved into the ancient geography of South America to find out just why the Amazon has so many fishes.
Two broad, global patterns of biogeography help explain Amazon fish diversity, but they don't tell the whole story. Size matters, and the Amazon River basin is a large, watery place, covering more than 2.5 million square miles, or 30 percent of the South American continent. In terms of water volume, no other river on earth comes close to it. In the rainy season, the Amazon discharges 3-6 million cubic feet of water per second into the Atlantic and accounts for 20 percent of the worldwide flow of freshwater into the oceans. So we might expect the vast basin, or watershed--consisting of the main stem of the Amazon and its thousands of tributaries--to contain many fish species. Yet the Orinoco, with a watershed area of less than half a million square miles, boasts at least 1,000 species, and even smaller rivers of the Guianas teem with hundreds of species. Size, then, is at best a partial explanation for the fish-rich Amazon.
One controversial theory that seeks to explain tropical America's biodiversity holds that climatic shifts starting about 2 million years ago and continuing through the Pleistocene Epoch, or until about ten thousand years ago, caused the repeated fragmentation and merging of tropical rainforests. Such shifts provided multiple opportunities for birds, butterflies, and plants to diverge and eventually become new species. But in the case of fishes, a time frame of 2 million years is just yesterday. Long before the Pleistocene, the Amazon teemed with species that are closely related to fishes alive today in the river. We need, then, to consider phenomena much older than 2 million years as we search for clues to the diversification and biogeography of Amazon fishes.
As early as the middle Miocene, about 15 million years ago, the tropical American fish fauna was essentially like today's. It included many living groups: stingrays, lungfish, pirarucu, piranha, goliath catfishes, some electric fishes, and cichlids. A primatologist friend and colleague once showed me a fossil of a 13-million-year-old fish he had excavated in Colombia. He had no idea what kind of fish he had found and was struck by its particularly large teeth. He was surprised when I was able to identify the fossil; the specimen looked just like a living fish I knew well, the tambaqui. Although no longer found in the rivers of central Colombia, the tambaqui, which feeds on seeds and fruits that fall from trees in the seasonally flooded forests, thrives today in the Amazon. Similarities between ancient and modern fishes crop up even further back in time. One well-preserved fossil fish about 59 million years old looks like, and is indeed related to, the Corydoras catfish popular today in the aquarium trade. And some fossil catfishes, lungfishes, and characins date back to the Late Cretaceous, 70 million years ago.
Long before the end of the Miocene, a great river, consisting of what are today the western Amazon and the Orinoco, flowed north through the lowland basin and into the Caribbean. About 10 million years ago, the rise of the eastern Andes created the Magdalena River basin in Colombia. Uplift continued until, by 8 million years ago, the mouth of the north-flowing Amazon-Orinoco was dammed. The river could no longer reach the Caribbean Sea. This major event in the geographical history of South America split the Orinoco and Amazon and reoriented their course from north-south to west-east. The Amazon began to flow into the Atlantic, as it does today. Over time, new rivers appeared in the north, and 3.5 million years ago the elevation of the Isthmus of Panama formed a bridge between the American continents.
As rivers appeared, disappeared, and changed course, their communities of fishes went along for the ride. The ranges of various species expanded, merged, or were disrupted. The geological upheavals that divided rivers and river basins provided opportunities for speciation when fish populations were isolated. The results are the species that today are found only in those systems. However, fossils from the regions now occupied by the Magdalena River and Lake Maracaibo show they once contained fishes no longer found there but that still inhabit the Amazon. These include lungfish, Arapaima, tambaqui, piranha, Hydrolycus, and goliath and pirarara catfishes. Although the fossil record documents such localized extinctions of living fish groups, we have no evidence of widespread extinctions and only two or three cases in the past 65 million years in which an Amazon fish species was completely extinguished.
Dark New World
The Amazon and its tributaries are not only vast but also fast moving, deep, and dark. In the clearer tributaries an observer may peer down to a depth of several yards, but in most of this siltladen river system, light does not penetrate even three feet. One of the main challenges faced by ichthyologists hoping to discover and document the fish species in very big South American rivers is to devise ways to survey less accessible parts of the channels. My colleagues and I found that from large boats or even motorized canoes, we could trawl depths of a few to more than 150 feet by casting a weighted net with a wide mouth and very fine mesh. In the process, we lost plenty of equipment as our gear snagged on submerged trees and rocks in water that was moving six or more feet per second.
So far on our Amazon collecting trips, we have netted some 240 species offish, some of them with surpassingly strange lifestyles. Especially well adapted to low or nonexistent light levels are several new species of electric fishes and catfishes. Many, such as the electric fish Orthosternarchus and the catfish Bathycetopsis, are blind or have only tiny vestigial eyes; some lack pigment. One transparent catfish, Micromyzon, has thick bones and armor plating but is among the tiniest of freshwater fish: an adult specimen carrying eggs measured just one-third of an inch long. Electric fishes of deep waters are able to navigate and locate prey using electric impulses. Each species has its own distinct patterns of electric discharges that it uses to communicate, much like birds use song. One electric fish bears a fleshy protrusion above its chin; we can only guess what that feature might be used for.
Some catfishes provide abundant food for the people living in the Amazon River basin and are often caught as adults in shallow water near river banks. But they appear to spend their juvenile stage floating and feeding near the surface of the river's central channels. Understanding such lifestyles is important for managing the harvest of food fish in the Amazon.
Two of the strangest creatures in this new realm are a pair of related electric fish species that we named Magosternarchus raptor and M. duccis. About ten inches long, these fishes make a living by feeding on the tails of other species of electric fish. We found this out when their stomachs proved to contain nothing but fish tails. Their victims can conveniently regenerate the nibbled appendage, renewing their own posteriors and the food source of the tail eaters. (We suspect that Magosternarchus may also dine on the tails of its own kind.) While we expect the Amazon and other big tropical rivers around the world to reveal more secrets in the future, some spots in these rivers run forbiddingly deep--up to 300 feet in the Amazon near Manaus, Brazil--and this may be deep enough to preserve some riverine mysteries.--J.L.
Ichthyologist John Lundberg ("Freshwater Riches of the Amazon" page 36) began studying the fishes of South America's Orinoco River in 1974 and those of the Amazon River in 1990. His pioneering surveys of the fish life of deep river channels have turned up many new species, including catfishes and electric fishes with unusual lifestyles. Lundberg, left, says his "innate attraction to fossils" has led him to broaden his focus; he uses insights from paleontology, geology, and biogeography in his work on tropical fish diversity. Formerly a professor at Duke University and the University of Arizona, Lundberg is now curator of ichthyology at the Academy of Natural Sciences in Philadelphia and holds adjunct professorships at the University of Pennsylvania and Drexel University.